В бд проверить работу
 ON DELETE SET NULL
 ON UPDATE CASCADE ..
  `author_book`
  (
    `author_id`          INT(11) UNSIGNED NOT NULL,
    `book_id`          INT(11) UNSIGNED NOT NULL,
    PRIMARY KEY (`author_id`, book_id),
    FOREIGN KEY (`author_id`) REFERENCES `author` (`id`)
      ON DELETE SET NULL

      - что если будет 2 NULL и т. п.?

